The GaN-based semiconductor light emitting device comprising the GaN-based semiconductor layer is considered to be a next generation lighting source replacing the fluorescent lamp. The GaN-based semiconductor light emitting device basically includes a pn junction diode in which a light-emitting layer is sandwiched between an n-type GaN-based semiconductor layer and a p-type GaN-based semiconductor layer. Electrons are injected into the light emitting layer from the n-type GaN-based semiconductor layer, holes are injected into the light emitting layer from the p-type GaN-based semiconductor layer, and they are recombined and emit light in the light emitting layer.
Conventionally, in a red, orange, or yellow LED, there is technology in which light is taken out with high efficiency from the light emitting layer of the LED by depositing a thick semiconductor current diffusion layer consisting of any one kind of GaP, GaAsP, and AlGaAs, which are materials that are transparent to the wavelength of each LED (which means to transmit light emitted in the LED and having its wavelength, below the same). On the other hand, in a LED made of a GaN-based material (GaN-based LED) that emits green, blue, purple, or ultraviolet light, there is a problem that the above-described materials are opaque to a light emission wavelength (a wavelength of the light emitted in the light emitting layer, below the same) and are not appropriate as a transparent current diffusion layer.
For the GaN-based LED that emits green, blue, purple, or ultraviolet light, the n-type GaN-based semiconductor layer or the p-type GaN-based semiconductor layer may be transparent, and the n-type GaN-based semiconductor layer also may function as the current diffusion layer. However, the p-type GaN-based semiconductor layer is not preferable because the thickness is normally 0.3 μm or less due to its high resistance and element resistance increases by thickening further. Therefore, it is not appropriate to use as the current diffusion layer flowing current effectively in the entire device.
Then, in the GaN-based LED, a transparent electrode layer is formed to cover substantially the entire surface on the top of the p-type GaN-based semiconductor layer. Generally, a metal electrode such as Ni and Au alloy is used for the transparent electrode, by forming an extremely thin layer as 0.1 μm or less. Thereby it is made to be transparent, although it is translucent, and possible to transmit the light from the light emitting layer. However, light extraction efficiency is poor.
Consequently, technology is known to improve the light extraction efficiency by using a transparent and conductive material such as ITO (indium tin oxide) or ZnO-based compound (that means oxides including Zn, for example, ZnO, oxides of a group II element, and/or oxides of a group IIB element) (for example, refer to Patent Document 1). Further, in Non-Patent Document 1, the inventors of the present invention actually showed technology in which the light emitting efficiency of the GaN-based semiconductor light emitting device is improved by using a ZnO transparent electrode layer doped with Ga. However, to use practically the LED for a lighting lamp or the like, the higher the light emitting efficiency of the GaN-based LED to be used as a white light source, the more preferable it is. However, under the present conditions, it is inferior to the light emitting efficiency of a luminescent lamp, and technology that further increases the light emitting efficiency of the GaN-based LED for practical use is necessary.
In the technology using the above-described transparent electrode layer, because thickness of the transparent electrode layer is a few μm or less, a part of the light generated in the GaN-based semiconductor light emitting layer that transmits in a vertical direction to the light emitting layer from each light emitting point is radiated from a surface to the outside. However, it becomes difficult to radiate to the outside of the element (which means semiconductor layers and electrodes of one chip, below the same), in case of lights which transmits to the direction tilted from the vertical direction, though they are most of the light emitted in the light emitting layer, they may be captured in the inside of the element by repeating total reflections in the element, and there is a problem that it is not radiated to the outside of the element.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-164570
Non-Patent Document 1: Japanese Journal of Applied Physics Vol. 43, No. 2A, 2004, pp. L180-L182.